100 GENERAL SCIENCE 



until a temperature of 4 C. is reached, when the water 

 begin, to expand again, and as it approaches the freezing 

 point becomes much lighter than water at higher tem- 

 peratures. As water is cooled after it becomes ice 

 it contracts like ordinary solids, but it never contracts 

 enough so that its volume is as small as when it was a 

 liquid. Since ice is lighter than water, it floats 

 on water and forms a covering which prevents 

 the water beneath it from freezing, thus keep- 

 ing it habitable for fish and other sea animals, 

 even in the coldest regions. 



If water is cooled until the temperature 

 reaches zero, ice will begin to form. As cold 

 is further applied, no change in temperature 

 will be noticed until the whole mixture is 

 frozen, when the temperature will again begin 

 to fall. The freezing point of water may be 

 lowered by dissolving salts of various sorts in 

 it. The water of the ocean freezes at a lower 

 temperature than the still water of our inland lakes. Can 

 you think of a reason why this is so? 



Steam. When water is sufficiently heated, it changes 

 to steam. The steam rises through the water in bubbles 

 and escapes at the surface into the air. When the forma- 

 tion of these bubbles occurs rapidly, the water is said to 

 boil. The temperature at which boiling begins is called 

 the boiling point. On the Centigrade scale this point is 

 100. On the. Fahrenheit it is 212. However, this 

 point varies with the air pressure. As the pressure on the 

 surface of the liquid is decreased, the temperature of the 

 boiling point is lowered. This change is readily noticed 

 in ascending mountains. On the top of Mont Blanc 

 the boiling point of water is 84 C. Water in Denver 



